Does it sound like a rattle or clack when first started? My Nissan Hardbody ('95, with the KA24E engine) would do this also. It'd sound like a diesel for a second or so. The general opinion was the timing chain system in the front was the culprit. Or more precisely, it was the network of chain guides and tensioners. It probably wasn't designed very well.
Oils higher in moly helped mine. The "old" formula Havoline seemed to do the best. At the time, I was a hardcore Valvoline conventional user, and that stuff had no moly in it. BITOGers convinced me to try the Havoline, and it really did help (5w30). This was probably 6 years ago.
I've known about a lot of Nissan engines with this type of noise upon startup. As many have said, it does not seem to affect engine life. That KA24E had 212k miles on it when my brother finally sold it. It's out of the family now, so I don't know how it's doing.